This paper describes a complex simulation of a pumped-storage hydroelectric power station. Dinorwig power station's fast response is essential for controlling the electricity supply frequency on the national grid in Great Britain. The paper describes the station's key physical features and it is shown that the plant is nonlinear, multivariable and time-varying. The details of a Simulink® model, which is valid over a wide range of operating conditions, are presented. The improved understanding of the station's dynamic characteristics is illustrated by using the simulation to investigate an incident of oscillatory behaviour in its power output.